Characterization of Binary Hydrates Containing Methane by X-ray Diffraction and Microscopic Laser Raman Spectroscopy

Abstract

BACKGROUND The crystal structure of natural gas hydrate mainly depends on the species and composition of guest molecules. At present, the structure and spectral characteristics of single-component hydrate are relatively clear, but the studies of multi-component hydrates are relatively scarce. OBJECTIVES To solve the problem of structure identification of multi-component hydrate, and understand its spectral characteristics. METHODS Methane-propane (CH4-C3H8) and methane-tetrahydrofuran (CH4-THF) binary hydrates, as well as CH4, C3H8 and THF (molar ratio 1:17) single-component hydrate samples were synthesized and characterized by low-temperature X-ray powder diffraction (PXRD) and Raman spectroscopy. RESULTS Both of the binary hydrates were typical structure Ⅱ hydrates, which were the same as those of C3H8 and THF hydrate. The crystal cell parameters a for CH4-C3H8 and CH4-THF binary hydrates were 17.2312×10-10 m and 17.2241×10-10 m, respectively. For CH4-C3H8 hydrate, CH4 was distributed in both large and small cages, showing two characteristic Raman peaks (2900cm-1 and 2911cm-1); C3H8 was only distributed in the large cage. Compared with C3H8 hydrate, the C-H stretching vibration peak was almost unchanged while the C-C stretching vibration peak (873cm-1) shifted by 3cm-1 to low frequency. For CH4-THF hydrate, the large cage was occupied by THF and CH4 was only filled in the small cage (2910cm-1). The peak positions of C-C and C-H stretching vibrations of THF in CH4-THF hydrate were consistent with those of THF hydrate. CONCLUSIONS The structure types of binary hydrates are consistent with those of single component hydrates. Molecules with larger sizes play a key role in the crystal structure of binary hydrates containing CH4. Moreover, the distribution of CH4 molecules in the cages are also influenced by larger molecules, and the Raman spectral characteristics of binary hydrates are significantly different. The conclusion of this study has important guiding significance for identifying the microstructure of multi-component hydrate based on spectral characteristics.

Keywords

• binary hydrate containing methane
• crystal structure
• spectrum characteristics
• guest molecule
• x-ray powder diffraction
• microscopic laser raman spectroscopy